1. Field of Invention
This invention is directed to systems and methods for monitoring and controlling current density delivered to a copy substrate by a transfer unit in electrostatic reproduction devices.
2. Description of Related Art
In a typical electrostatic reproduction process, reproduction is initiated by selectively charging and/or discharging a charge receptive imaging member (hereinafter “receptor”), e.g., a photoreceptor, in accordance with an original input document or an imaging signal, thereby generating an electrostatic latent image on the imaging member. This latent image is subsequently developed into a visible image by a process in which a charged developing material is deposited onto the surface of the latent image bearing imaging member. The charged particles in the developing material adhere to image areas of the latent image to form a visible developed image corresponding to the latent image on the imaging member. The developed image may be subsequently transferred, either directly or indirectly, from the imaging member to a copy substrate, such as, for example, paper or the like, to produce a “hard copy” output document.
Image transfer between the imaging member and the copy substrate is facilitated by passing the copy substrate through a transfer unit in the electrostatic printing device and imparting an electrostatic charge to the copy substrate. This electrostatic charge in the copy substrate allows for image transfer to, and image fixing, or “tacking” of the developing material on, the copy substrate. Specifically, the copy substrate is passed between a current generation unit, such as, for example, a voltage shield, and a receptor unit that faces the current generation unit. The receptor unit is bonded to a substrate, which generally forms a ground plane electrically grounding the receptor. Electrical current passing between the current generation unit and the grounded receptor unit electrostatically charges the copy substrate.
Electrostatic reproduction devices allow for different types of copy substrates, i.e., substrates of differing width and/or thickness and substrates having differing characteristic electrical resistivity. When the copy substrate is not as wide as the total width of the charge exposed area of the receptor, there are areas of the receptor that are exposed directly to the current generation unit without the protection of the resistivity associated with the copy substrate between the current generation unit and the grounded receptor. When a portion of the receptor is left exposed directly to the current generation unit, a phenomenon called “End Leakage Current Effect” results, whereby a highly positive voltage, and a resultantly high proportion of the total dynamic current produced in the transfer unit, can be found in the exposed portion of the receptor surface rather than in areas contacted by copy substrate. The extent of the end leakage current effect depends on, among other variables, the width of the copy substrate to which the image is being transferred.
Conventional electrostatic reproduction devices monitor, and control as constant, total dynamic current produced in the transfer unit. The current density in these devices, which is the current per unit length going to the copy substrate, is a function of many variables, which include width of the copy substrate and end leakage current effect. When copy substrate width changes, with total current kept constant, the current density to the copy substrate changes, i.e., drops when the copy substrate width decreases and rises when the copy substrate width increases.
It is advantageous to keep current density to the copy substrate constant. When electrostatic reproduction devices, however, control only total dynamic current between the current generation unit and the receptor, which is a sum of the current density delivered to the copy substrate and the current density going to regions beyond the copy substrate that for ease will be referred to as no-paper regions, current density to the copy substrate changes with changes in characteristics of the copy substrate. Conventional electrostatic reproduction devices control only average current density often by varying the voltage (Vshield) applied by the current generation unit opposite the receptor to maintain constant total dynamic current (Ldy). As width of the copy substrate changes, exposing more or less of the receptor directly to the total dynamic current for narrower and wider substrates respectively, the voltage is increased or decreased to keep the total dynamic current constant. A fundamental difficulty is that where the total dynamic current flows depends on whether there is a copy substrate, with certain characteristic resistivity, present over the receptor. The goal is to control the current density of the current applied to the copy substrate as this variable is ultimately related to the electrostatic forces trying to transfer toner and trying to electrostatically tack images to copy substrate surfaces.
To obtain the same charge density on the copy substrate as in the no-paper region, current applied to the copy substrate needs to be higher than current applied to the no-paper region because of resistivity of the copy substrate. The deposited charge is further removed from the ground plane of the receptor in the region covered by the copy substrate than it is in the no-paper region. Because the voltage potential difference is lower at the copy substrate, the current density is necessarily lower in the copy substrate region.
Conventional electrostatic reproduction devices begin operation by supplying a certain voltage. The total dynamic current is measured, and feedback is provided to adjust the voltage applied to maintain a preset total dynamic current between the current generation unit and the receptor. Total voltage required to produce a set dynamic current averaged across the regions of the receptor that are covered by copy substrate and the no-paper regions decreases as the width of the copy substrate decreases and exposes more no-paper region of the receptor. The voltage the system chooses if the width of the copy substrate is very narrow is much smaller than the voltage it chooses if the copy substrate is very wide with respect to the total width of the charge exposed area of the receptor, which is fixed. Therefore, the current density is much smaller when the copy substrate is narrow than it is when the copy substrate is wide.
There is certain latitude to the acceptable current density in a copy substrate based on the properties of the copy substrate relative to the transfer phenomenon. Latitude refers to an acceptable range of the electrostatic force applied to a copy substrate to facilitate pulling toner off the receptor and sufficient to electrostatically tack an image to the copy substrate. Latitude defines the limits that the electrostatic reproduction device needs to create regarding sufficient electrostatic field in a particular copy substrate to support the electrostatic reproduction process. With narrow copy substrate relative to the width of the charge exposed area of the receptor yielding a decrease in voltage to maintain total dynamic current, the system may not provide the current density through the copy substrate to meet the latitude required. The effective electrostatic transfer field between the copy substrate and the receptor decreases to an unacceptable level. Latitude in transfer systems depends on toner properties and a number of other variables. For instance, exceptional toner adhesion properties may yield wider latitude, allowing the device to accept significant decreases in the effective electrostatic force delivered to and through the copy substrate. There is, however, in all systems a threshold below which the current density of the current applied to the copy substrate will not support acceptable electrostatic image transfer. There is also conversely a threshold level above which the current density of the current applied to the copy substrate will begin to create unacceptable defects on the print such as those typically related to air breakdown effects. Latitude in the transfer system refers to current density conditions between these extremes. In general, when latitude is considered acceptable, it is understood that there may be some degradation in image quality under certain conditions, but such degradation is acceptable in the electrostatic reproduction device, e.g., not substantially noticeable to the naked eye.
Complex solutions to controlling current density in a copy substrate include segmenting a current generation unit of the electrostatic reproduction device. Current density is sensed and monitored through each of the individual discrete segments. Applied voltage is adjusted only to those segments that the sensing and monitoring functions determine are within the width of the copy substrate. The current density to the copy substrate, therefore, is maintained at constant value while the current to the areas of the receptor where there is no copy substrate is turned off. A disadvantage of this solution is that such a solution requires a special segmented voltage supply or current generation unit, which includes multiple connections to a power source and additional switching, both of which could be complex.